Vacuum conveying systems are in widespread use in both industrial and agricultural establishments for handling dry bulk materials such as powders and pellets. These conveying systems can move dry bulk materials through closed dust-free systems at rates ranging from a few hundred pounds per hour up to virtually any requirement. Vacuum storage hoppers form an important part of such systems. A vacuum storage hopper is usually installed over a larger storage bin or over processing equipment where it directly feeds a machine hopper. In such cases, the machine hopper is usually equipped with a level control to maintain an adequate supply of the dry bulk material for the machine.
In order to move the dry bulk material through a vacuum conveying system, a vacuum is created in the vacuum storage hopper. The vacuum may be created in any conventional way but it is usually by means of a vacuum line connection to the inlet side of a positive displacement air blower. The vacuum so created draws free flowing dry bulk material into the vacuum storage hopper through a material feed tube or pipe. A filter in the vacuum hopper cover prevents the dry bulk material from being drawn into the vacuum line. When the vacuum storage hopper is full, a timer or level sensor signals a directional control valve to switch from vacuum to air under pressure for delivery to the vacuum storage hopper. A gate or closure member in the bottom of the vacuum storage hopper is opened to discharge the material from the hopper while a flapper valve on the material feed tube closes. The pressurized air entering the hopper cleans the filter after each loading cycle and aids in emptying the material through the discharge gate in the bottom of the hopper.
Conveying systems using vacuum storage hoppers are frequently fully automated. The powders or pellets moving through the system sometime coagulate in the hopper, thereby "bridging" or obstructing the discharge outlet. When this happens, the processing machine feed is cut off and the processing machine is automatically stopped. An attendant must then diagnose the trouble and correct the cause of the interruption of material flow. The problem of "bridging" of coherent material in the discharge of storage hoppers has long been recognized and various solutions have been proposed. One readily apparent solution is to increase the pressure of the air introduced into the vacuum storage hopper to force out the material. Safely requirements, however, limit the pressure of the air that can be introduced in the hopper and this method alone has not been successful.
Another proposed solution to this problem involved the use of a flexible liner in the storage hopper which could be flexed to break up obstructions. U.S. Pat. No. 3,346,917 to Lennox shows a flexible liner 33 installed in the discharge cone of a hopper. The liner is sealed at its top and bottom to the hopper wall. Air under pressure is introduced in the space between the liner and the hopper wall 24 through a valve 37 and a conduit 36. A control 38 for the valve 37 limits the inward flexure of the liner and the flexing of the liner is used to release jams and bridging.
Another approach to the problem of "bridging" is shown in U.S. Pat. No. 3,115,369 issued to Bozich. A membrane 36 is installed as a liner for the conical wall portion of a hopper and the membrane is sealed at its top and bottom edges to the hopper wall. The membrane is porous so that air can be forced through it to aerate the contents of the hopper. Air can also be introduced at the top of the hopper. However, an interlock is provided to prevent air from being introduced simultaneously into the top of the hopper and between the hopper wall and membrane.
Davis U.S. Pat. No. 2,732,099 and Hartman U.S. Pat. No. 3,079,018 both disclose hoppers that utilize plural air inflated "bumpers" in the bottom portions of the hopper cones to avoid "bridging." The Davis patent expressly provides for periodic inflation and deflation of the bumpers.
Carter U.S. Pat. No. 2,756,906 has an internal liner 31 in the cone of a hopper, rather like that shown in the Lennox patent, with a plurality of sealed rubber tubes 36 located between the liner and the hopper wall 30. The space between the liner and the wall 30 is periodically evacuated through a vacuum line 37.
Emery et al U.S. Pat. No. 3,367,723 shows an aerating device that uses a rigid perforated wall 22 to introduce air into the contents of a hopper from the bottom thereof.
None of the previously described mechanisms or methods have been completely successful in preventing jamming or bridging of powders or pellets in the discharge outlet of vacuum storage hoppers.